The present invention relates to a method of constructing an optical wiring network of optical fiber cables for connecting optical information processors on many floors of a building by feeding an optical fiber cable through a tube using a compressed gas provided by a pressure feeding apparatus.
A method of guiding optical fiber cables through tubes with an air stream was developed by British Telecom Research Laboratories, and cable installation by this method is described in an article entitled "The Blown Fiber cable", IEEE Journal on Selected Areas in Communications, Vol. SAC-4, No. 5, August 1986, pp. 679-685. According to the theoretical consideration made in this article, when an optical fiber cable is drawn in a horizontal direction using an air stream, the "effective drag force" per unit length, which is equal to the hydrostatic force due to the simple pressure drop plus the purely viscous component due to the frictional shear force at the surface of the fiber cable, is constant in the longitudinal direction given a constant pressure drop within the tube, and hence the total effective drag force increases in proportion to the installed length of the optical fiber cable. That is, the effective drag force per unit length, f, acting on the optical fiber cable with the air stream is expressed by the following equation: EQU f=.pi.r.sub.1 r.sub.2 (dP/dL) (1)
where r.sub.1 is an outer radius of the cable, r.sub.2 is an inner radius of the tube, and dP/dL is a coefficient representing the gradient of pressure drop within the tube.
In the aforementioned paper, since the gradient of pressure drop within the tube is assumed to be constant along the tube, equation (1) may be rewritten as: EQU f=.pi.r.sub.1 r.sub.2 (P.sub.0 /L.sub.t) (2)
where P.sub.0 is a pressure at the entrance end of the tube, and L.sub.t is a total length of the tube.
If equation (2) holds, the effective drag force per unit length, f, depends on neither the installed length of the optical fiber cable nor the distance from the entrance end of the tube, but is determined by r.sub.1, r.sub.2, P.sub.0 and L.sub.t and remains constant in the longitudinal direction of the optical fiber cable.
If the wiring route contains an ascending portion, the weight of the optical fiber cable drawn into the tube provides a resistance against the drag force by the air stream and the state of the feeding operation is determined by whether the sum of the resistance due to the cable weight and the force of friction between the optical fiber cable and the inner surface of the tube is greater than the drag force. Hence, if r.sub.1, r.sub.2, P.sub.0 and L.sub.t are set at such values that the drag force per unit length, f, is greater than the sum of the resistance and the frictional force, the optical fiber cable can be drawn and installed into the tube over its entire length. In this case, the calculated force per unit length depends on neither the distance of the ascending portion from the entrance end of the tube, nor the installed length of the optical fiber cable into the tube.
In practice, however, the drawing speed of the optical fiber cable decreases as the cable is drawn into the tube over an increased distance and, in an extreme case, the cable may get stuck in the middle of the tube. In this regard, the authors of "The Blown Fiber Cable", supra, stated that the measured values of tension of an optical fiber cable drawn into a tube using an air stream did not agree with those of theoretical considerations, admitting that theoretical considerations do not necessarily reflect the reality.
If the wiring route contains an ascending portion, the profile of drawing speed varies with the distance from the entrance end of the tube and the drag force per unit length of the optical fiber cable would not be uniform along its length. In the case where the tube is provided in a wiring route containing an ascending portion, the optical fiber cable often fails to be effectively drawn and installed into the tube using an air stream.
In order to insure positive drawing of the optical fiber cable, the following practice has been adopted in the prior art: the wiring route is divided into shorter sections (tubes); part of the optical fiber cable is drawn and installed into a divided tube of the route; the remaining part of the cable is rewound at the exit end of that tube; and the remaining part of the cable is drawn into the next tube using an air stream. This method, however, is not highly efficient since cable insertion, installation and rewinding operations have to be repeated for each divided section of the wiring route. Further, an optical fiber cable may have to be spliced at many portions at the sacrifice of its transmission characteristics.